When you enable binlogs in the my.cnf file you can either set the log-bin flag to true, or you can set it to a path and file name prefix such as this:
This changes the default location where binlogs are stored. The problem is that when you connect to mysql there is currently no way to query the server to find out if that path has been changed, and what it currently is. This means you can't be sure where any server's binlogs are actually stored.
This is such a trivial change to expose that info somewhere. Why neglect such basic information?
The relay_log variable which behaves the same in the conf file, shows the behavior you would expect in the global variables on the server -- the same as the value you set in the conf.
If MySQL is worried about backwards compatibility then please just add a new variable name.